Repair and Maintenance of a Drascombe Lugger

Making a Spar Gauge

The first stage in converting a square plank into a round
spar is to plane down the square edges to make the
cross-section into that of a regular octagon. A spar
gauge is a simple device which will mark two lines on the
face of each side of a square plank (either straight or
tapering), enabling the four corners of the plank to be
planed down to these lines to produce the required
octagonal shape. This page outlines the geometrical
theory underlying the spar gauge and then gives guidance
on construction.

How a Spar Gauge Works

Spar Gauge Geometry

The top sketch represents a cross-section of a
square plank of width (w), before shaping
into a spar. The first stage is to plane off the
corners of the square to form a regular
octagon (shown in black outline), with sides of
identical width (x). The sides (x),
in each corner of the square, form the hypoteneuse
of four 45-90-45 triangles with the other sides
(y) of the triangles being of equal length.
Most importantly as regards a spar gauge, the
ratio x : y will always be 1.41 : 1.

The lower sketch shows a spar gauge
consisting of a rectangular piece of wood with two
outer guides (represented by red circles) at each
end (placed either side of the spar), and two
pencils (also represented by red circles) fixed in
position either side of the centre point of the
gauge. The gauge is placed across the side of the
square spar and twisted in the direction of the
upper right and lower left red arrows so that the
guides are in contact with the sides of the spar.
Keeping the guides in contact with the sides, the
gauge is moved in the direction of the red (down)
arrow to scribe the two lines delineating the
corners of the regular octagon. The spacing between
the guides and the pencils simply makes use of
the1.41 : 1 ratio (described above) to mark
out the extent of a regular octagon on the square
sides of the unshaped-spar.

The gauge will correctly mark the position of
the octagonal corners on the face of any square
spar which will fit between the two outer guides,
as long as the guides are both in contact with the
spar sides as the gauge is moved down the spar.

The most useful derivative of the 1.41 : 1
ratio is 10 : 7.07 (shown in the sketch
above), as this translates very neatly into
1¾" to 2½" with an
overall distance of 6" between the inner
faces of the guides - an ideally sized spar gauge
for marking out Drascombe spars.

Calculation by Formula

This table (below) uses a formula based on the
1.41 : 1 ratio to calculate the positions
of the octagonal corners on a square spar. Enter
a value for spar width w and click on
either of the cells adjacent to x or
y to calculate the corresponding values.
(See the sketch above for the positions of
w, x & y.)

When w =

x =

y =

Making the Spar Gauge

Having looked at the geometry of how a spar gauge works,
this section gives a plan from which a spar gauge can be
constructed.

CONSTRUCTION NOTES
A piece of wood (hardwood or softwood) with flat planed
faces and measuring about 8" long, 2" wide and
¾" thick can be used to make the gauge. The
holes for the pencils and outer guides are drilled along
the centre line of the wood. Note that on the plan,
the distances between the pencil and guide holes is from
the centre of the pencil holes to the inside
faces (NOT THE CENTRE), of the guide holes. The
¾" thickness of the gauge body is to make it
easier to ensure that the guides and pencils are held at
90° to the upper & lower faces of the gauge and
to this end the holes should be drilled as vertically and
as true as possible.

The two outer guides can be made out of round
wooden dowel (available in most hardware shops or from
timber merchants). A diameter of 3/16" is
suggested but can vary according to what is available.
The length can be 2¾", the top ¾"
being glued into the corresponding hole in the body of
the gauge.

The two pencil holes have a distance between the
centre points of 2½" with a suggested
diameter of 5/16" which corresponds to the
diameter of a typical pencil.

A Completed Spar Gauge

A Completed Spar Gauge

This spar gauge is based on the diagram above but
the two outer guides are made out of 5/16" diameter
aluminium tube, and the two pencils are contained
in 3/8" aluminium tube.

Using a Spar Gauge

The Spar Gauge is placed across the plank and
rotated sideways so that the two guides at each end
are in contact with the plank sides. They must
remain in contact as the gauge is moved along the
length of the plank, with the two pencils marking
the two lines as shown. Repeat for each of the four
sides of the plank.